The present invention relates to integrated circuit structures and fabrication methods.
Background: Aluminum Texture and Electromigration
Recently, contact and via filling with aluminum alloys has attracted a great deal of attention. Compared with contact/via filling with chemical vapor deposition (CVD) tungsten, aluminum filling has the advantages of lower cost, higher yield, and potentially better electromigration resistance (since there is less flux divergence near the plug).
There are two main categories of aluminum filling processes, one with elevated temperature and another one with high pressure to enhance the aluminum transport to achieve complete filling of cavities (via, contacts, or trenches). Both methods require a degas step prior to liner deposition, to desorb the moisture, oxygen, or other volatile species from the dielectric on the sidewall of contacts and vias. Inadequate degas will hamper the filling process. Furthermore, an etch step is essential to clean the bottom of contacts or vias. This step removes residues from the contact/via etch step and/or the native oxide formed on the lower level metallization after being exposed by the contact/via etch step and thereby lowers the contact resistance.
Electromigration is one of the long-standing liabilities of aluminum metallization. Nevertheless, some improvement can be achieved by using aluminum which has crystal grains oriented primarily in the &lt;111&gt; direction. See e.g. Shibata et al., "The Effects of Al &lt;111&gt; Crystal Orientation on Electromigration in Half-Micron Layered Al Interconnects," 32 JAPANESE J. APPL. PHYS. 4470 (1993); Kondo et al., "Effects of Grain Size and Preferred Orientation on the Electromigration Lifetime of Al-Based Layered Metallization," 78 J. APPL. PHYS., 6534 (1995); and Tracy et al., "Texture in Multilayer Metallization Structures," 76 J. APPL. PHYS. 2671 (1994), all of which are hereby incorporated by reference.
Thus the formation of aluminum alloy thin films with a strong &lt;111&gt; texture enhances the electromigration resistance. The crystal-line orientation ("texture") of polycrystalline aluminum alloy films depends on the deposition parameters, thermal history, and on the liners underneath. Titanium and TiN are the most commonly used liners. Currently, in aluminum via/contact filling processes with high pressure extrusion, poor aluminum texture has resulted in a wider electromigration (EM) lifetime distribution, which is undesirable. This poor aluminum texture is due, in part, to poor TiN texture under the aluminum film. Therefore, in order to reduce electromigration, it is necessary to improve the texture of titanium and TiN, toward titanium &lt;002&gt; and TiN &lt;111&gt;, to produce an aluminum film with a &lt;111&gt; texture.
There have been several proposed methods to optimize the deposition parameters of titanium and TiN to improve the texture. One such proposed method suggested that improved Ti &lt;002&gt; can be obtained if the titanium film is deposited at a low temperature. Although low titanium temperature can improve the texture, adequate wafer heating during liner deposition is required to improve the film's uniformity, lower the resistance, and reduce the stress. Another known method found that the texture of titanium films also depends on the surface roughness of various oxides. After chemical mechanical polishing (CMP) to reduce the surface roughness of oxide, dramatic improvement in the titanium preferred orientation was observed. Another proposed solution suggested that by argon sputter etch, the titanium preferred orientation can be improved, in addition to the texture of aluminum in the aluminum/titanium and aluminum/TiN/titanium metal stacks. However, in this solution, thermal oxide was used as substrate, and due to the high process temperature required (typically greater than 800 degrees C), thermal oxide cannot be used as an interlevel dielectric where aluminum is used as a metal interconnect.
Method to Improve Texture
The present application discloses a new method to improve the texture of titanium and aluminum, which will reduce electromigration by controlling the deposition conditions and the texture of the substrates. Aluminum films can develop strong &lt;111&gt; textures, when titanium is used underneath aluminum. However, to prevent the interaction between aluminum and titanium, a layer of TiN or other barrier is necessary. Fortunately, TiN has an atomic arrangement on the &lt;111&gt; plane which is similar to those of aluminum &lt;111&gt; and titanium &lt;002&gt;. Therefore, by controlling the orientation of titanium using a pre-sputter argon etch and low titanium deposition temperature, the texture of titanium can be transferred to TiN, and subsequently to the top metal layer. This method is particularly useful for advanced aluminum or copper metallization applications.
The advantages provided by the innovative method of the present invention include:
manufacturable; PA0 effective in improving titanium and aluminum textures; PA0 particularly effective in improving the texture of aluminum in the field; and PA0 compatible with current process flows.